where \(k=|{\bf k}|\)  is the frequency of the massless field.
Finally, the initial conditions for the perturbations will be chosen
according to the quantum fluctuations of free fields. The spectrum
is identical to that of a scalar quantum field in Minkowski space
[1]},
\(h(x,\eta ) = h(\eta )\,e^{\pm i{\bf k}.{\bf r}} \,\,,\,\,\qquad h(\eta ) \propto \frac{e^{\pm ik\eta }}{\sqrt{2k}} \,.\) 
